Summary of work: Genomic instability arises from several sources. These include problems with DNA repair, transcription and replication. They also include deficiencies in the topological metabolism of DNA which is controlled by a number of DNA metabolic enzymes. A group of these are topoisomerases, and we have been interested in their function as it relates to the handling of DNA damage. Several recent studies have shown that human topoisomerase I (htopoI) can recognize various DNA lesions and thereby form a covalent topoisomerase I-DNA complex, which is known to be detrimental to cells. We have investigated whether htopoI recognizes another htopoI that is covalently trapped on a DNA substrate. For this purpose we created an artificial DNA substrate containing a specific topoisomerase I binding sequence, where the enzyme was trapped in the covalently bound form. We demonstrate that, in vitro, free htopoI stimulates the formation of an additional cleavage complex immediately upstream of the covalently bound topoisomerase I. The predominant distance between the two cleavage sites is 13 nt. In addition we find that these two enzymes may form direct protein-protein contacts and we propose that these may be mediated through the formation of a dimer by domain swapping involving the C-terminal and the core domains. Finally, we discuss the possibility that the double cleavage reaction may be the initial step for the removal of the recognized cleavage complex. We also study genomic instability at the population level and how this changes with aging. This is done in DNA from humans enrolled in the Baltimore Longitudinal Study on Aging. Here we measure a number of polymorphisms in DNA repair genes to determien if there are significant changes with aging or with age-associated diseases.